In recent years, in an air conditioning system of a ventilation fan or an air conditioner, to optimize a status of a room or the air conditioning system, it is required that an air flow (air volume) be controlled to become a predetermined air flow value even though a static pressure changes due to clogging of a filter or a state of a vent provided in an air outlet.
For example, as a technique for controlling the air flow such that the air flow becomes the predetermined air flow value, for example, Patent Literature 1 is known. According to the conventional technique disclosed in Patent Literature 1, a command speed of a motor for driving a fan is calculated from a formula (1).S*=S+K×(S−Sa)  (1)
S* indicates the command speed of the motor, S indicates a motor speed, Sa indicates a target motor speed, and K indicates a gain. The target motor speed Sa is calculated from a formula (2).
                    Sa        =                              ∑                          n              =              0                        j                    ⁢                                          ⁢                      (                                          ∑                                  m                  =                  0                                i                            ⁢                                                          ⁢                              (                                                      k                    nm                                    ⁢                                      T                    n                                    ⁢                                      Q                                          *                      m                                                                      )                                      )                                              (        2        )            
T indicates motor torque, Q* indicates a predetermined air flow value, knm (n=0, 1, 2, . . . , j, m=0, 1, 2, . . . , i) indicates a constant, i indicates a finite value, and j indicates a finite value. The formula (2) represents a relationship in which the motor speed required to allow the air flow to become the predetermined air flow command Q* in a case where the motor torque is T is Sa.
Hereinafter, a description will be given of control of the motor in the case where a motor controller disclosed in Patent Literature 1 controls the air flow of the fan such that the air flow becomes the predetermined air flow value Q*, with reference to FIG. 7.
In FIG. 7, a curve F indicates a motor speed and motor torque at which the air flow becomes the predetermined air flow value Q* (hereinafter this curve will be referred to as “constant air flow curve”). A curve C indicates an environment curve unique to an air conditioning environment in which the motor is mounted (hereinafter will be referred to as “air conditioning environment curve”).
It is supposed that the operation point of the motor is a point A1 on the air conditioning environment curve C. In this case, the target motor speed Sa calculated from the formula (2) is a motor speed Sa1 at a point B1 on the constant air flow curve F at which motor torque T1 corresponding to the operation point A1 is obtained.
According to the formula (1), a difference (indicated by ΔS in FIG. 7) obtained by subtracting Sa1 from the motor speed S1 is multiplied by the gain K and the motor speed S1 is added to the resulting value, thereby resulting in a command speed S* (not shown) of the motor. Then, the command speed S* is output. The motor speed is controlled in accordance with the command speed S*, and thereby the operation point is shifted.
At the shifted operation point, a new command speed S* is calculated according to the formula (2) and the formula (1). This is repeated. The operation point is shifted to an operation point A2 and then to an operation point A3 in FIG. 7. Finally, obtained is an operation point at which the difference between the motor speed S and the target motor speed Sa becomes zero, i.e., an intersection E of the air conditioning environment curve C and the constant air flow curve F.
As described above, the motor controller disclosed in Patent Literature 1 is configured to calculate a compensation value required for the motor speed using the motor torque and the predetermined air flow value and the command speed is output based on the compensation value. This makes it possible to control the air flow such that the air flow becomes the predetermined air flow value, even if the predetermined air flow value changes or the static pressure changes due to clogging of the filter or opening/closing of the vent.